Forming of metal or polymeric pans is priorly accomplished by using the fixed or constant volume mold. That is, the general shape of the product is, for the most pan fixed by the mold construction and cannot be varied as one desires. The induced mechanical pressure and temperature are controlled in the molding process such that the product would acquire the desired microstructure, mechanical and/or fracture properties in addition to the appearance and geometric configuration. In situations where the speed and volume of production are of importance, injection molding is a choice adopted in practice. However, a limitation in the choice of product configuration has been found.
The rate of change of the volume with respect to the surface area, if not carefully controlled, could introduce defects and/or cracks in the formed products which are unacceptable. The rate at which the injected material accumulates locally must be controlled to minimize the excessive buildup of residual stresses and/or strains. Specimens with a large volume-to-surface area ratio are problematic because they are more vulnerable to cracking or break on account of non-uniform expansion or shrinkage. The shortcomings of the conventional mold for molding a product are summarized as follows:
1. The property of the molded product is poor; PA1 2. The adjustments of imposed pressure and heat for molding are difficult to control; PA1 3. The appearance and the outer surface of the product are rough and coarse; PA1 4. The shape of the mold is fixed and limited in configuration; and PA1 5. The manufacturing costs are raised due to a large ratio of imperfection of the products.
To free from forming stress-concentrating sites within a product during molding, and to produce a product having a constant or variable cross-sectional area, this invention was accordingly developed with these objectives in mind and is disclosed hereinafter.